Removal of contaminants

ABSTRACT

PCT No. PCT/AU90/00250 Sec. 371 Date Dec. 6, 1991 Sec. 102(e) Date Dec. 6, 1991 PCT Filed Jun. 7, 1990 PCT Pub. No. WO90/15024 PCT Pub. Date Dec. 13, 1990.A method of recovering contaminants from suspension or solution in a liquor comprises the steps of forming a foam of the liquor, displacing said foam onto a drainage device to dry said foam and separate the liquor said contaminants being retained in the dried foam and the drained liquor being reduced in contaminant content.

This invention relates to the removal or recovery of contaminants suchas metals or organic chemicals from solution or suspension.

With an increased emphasis on environmental safety and pollution controlthe efficient removal of contaminants from wastewater has became ofvital importance to the manufacturing industry.

Metals, particularly heavy metals, are one of the insidious pollutantsof our environment. They are not biodegradable and may exist in a numberof forms associated with living organisms, water, sediments andsuspended matter. The heavy metals accumulate in sediments and aresubsequently taken up by organisms. With the process of bio-accumulationthrough the food chain many edible species of aquatic life are affectedto the extent that their consumption may be hazardous.

Also the removal of oils and other organic chemical residues from wastewater streams is critical to efficient pollution control.

The most widely used method of treatment of waste water containing heavymetals is by precipitation and sedimentation. Although this treatmentmethod can, when operating at peak efficiency, give an effluentcontaining less than 1 ppm of each heavy metal, under normal operatingconditions it often exceeds the limit (e.g. 10 ppm) for disposal tosewer set by Trade Waste Laws. The reasons for this range from badmaintenance routine to the presence of interfering substances such asoils.

Sedimentation is a slow process requiring large settling volumes.

The conventional process is also subject to number of interferenceswhich can effect the rate of settling--particularly if the system usesNaOH for neutralization. Even well operated clarifiers can have 5 to 50ppm of suspended solids in the overflow. Contaminants, such as oils andsubstances which produce gases, are the most common source of problemssince they interfere with the sedimentation process by increasing thebuoyancy of the particles.

The flocculants used to speed up the sedimentation process inconventional technology are almost exclusively organic polyelectrolytes.These are expensive and require a certain degree of mixing beforeclarification is attempted. Depending on the size of the clarifier used,a separate mixing zone may be required in the process. Thepolyelectrolytes also add to the volume of sludge generated.

The polymer flocculants often required pH values of 9 to 10 for peakperformance. Although these pH values are acceptable for disposal tosewer (max. pH=10) a considerable expense is incurred in raising the pHto this level.

The conventional technique of precipitation results in gelatinousprecipitate that, even after flocculation and sedimentation yields a wetbulky sludge (95% water) that requires disposal. One of the mostpromising methods of separation and concentration of metal ions and fineparticles is adsorbing colloid flotation.

Adsorbing colloid flotation has a number of attractive features (i) lowenergy requirements (ii) high removal efficiency (iii) reasonablecapital requirements and (iv) comparatively low maintenance andoperating costs, thereby potentially providing a low cost method ofheavy metals recovery from industrial wastewaters.

The process generally involves the production of a hydroxide precipitateof the metal ions via pH adjustment by adsorption and/orco-precipitation with a floc generating material such as Fe(OH)₃ orAl(OH)₃, rendering the floc hydrophobic by adsorption of a surfactantand its subsequent removal by flotation with air bubbles.

Batch flotation yield high levels of metal removal for most ioncombinations but with a time requirement of up to 10 minutes. Similarlylaboratory studies of copper recovery using iron hydroxide as theadsorbing colloid and sodium lauryl sulphate as the frother requiredabout 25 minutes but achieved levels in the effluent consistently lessthan 1 ppm. Most studies of heavy metal removal using these techniqueshave been carried out on simulated wastewaters made up from purecomponents.

Two studies on actual industrial effluents were based on copper removalfrom a copper smelter wastewater and chromium removal from aneletroplating effluent. Again flotation times of about 5 minutes wererequired to achieve residual levels less than 1 ppm.

It is an object of this invention to provide an efficient means ofremoving or recovering contaminants from suspension or solution whichalso lends itself to small scale economic operation.

To this end the present invention provides a method of recoveringcontaminants from suspension or solution in a liquor which comprises thesteps of forming a foam of the liquor, displacing said foam onto adrainage device to dry said foam, and separate the liquor saidcontaminants being retained in the dried foam and the drained liquorbeing reduced in contaminant content.

In contrast to conventional froth flotation the present inventionproduces a foam which carries the majority of the liquor, with it overthe top of the foaming tank onto a drainage device. The foam flows overthe drainage device and dries, that is it is dewatered by drainageresulting in separation of the liquor from the foam. The dried foam maythen be collected and disposed of. The liquor reduced in contaminantlevels is collected from the drainage device.

Accordingly in a preferred embodiment of the invention we provide amethod of recovering waste water contaminants from a liquor such aswaste water comprising the following steps:

feeding the liquor into a tank; forming a foam by passing gas throughthe liquor such that greater than 50% by volume of the liquor fed intothe tank is displaced by overflow from the tank in the form of a foam;draining the foam; and collecting the liquor drained from the foam saidliquor having reduced contaminant levels.

Generally we have found a continuous process whereby liquor iscontinuously fed into the tank and foam continuously formed is mostpreferred although a batch process may be used if desired. Preferably atleast 80% by volume of the liquor is displaced by overflow in the formof a foam and more preferably at least 95% by volume.

Typically we have found that excellent results are achieved bydisplacing essentially all of the liquor via the overflow in the form offoam.

This invention generally achieves much faster throughput and lowerresistance times in the foaming tank with a consequent saving in plantsize.

The foaming step in the process of the invention is generally carriedout on the liquor in the presence of a collecting colloid andsurfactant.

The degree of removal of the contaminant is partly predicated on thechoice of collecting (or adsorbing) colloid. Typically the adsorbingcolloid is a metal hydroxide or metal sulphate of low water solubilityand preferred adsorbing colloids are generally selected from hydroxidesand sulfates of iron and aluminium and mixtures thereof. Preferredadsorbing colloids are Fe(OH)₃, Fe(OH)₂ and Al(OH)₃ particularly in thecase of heavy metal contaminants.

The level of adsorbing colloid is typically in the range of from 1 to1000 ppm and preferably in the range of from 5 to 500 ppm.

Most preferably the adsorbing colloid comprises a mixture of Al(OH)₃ andone or both of Fe(OH)₃ and Fe(OH)₂ which is preferably in a molar ratioin the range of from 5:1 to 1:5 and most preferably in the range of Alto Fe of from 1;1 to 1:4.

It will be appreciated that the absorbing colloid may be formed in situunder the appropriate conditions. Preferably the surfactants used areselected to provide a persistent, rigid, elastic foam. It is desirablethat the foam persists for a sufficient period to allow efficientdraining of the liquor from the foam and good retention of contaminants.Typically the foam will persist for at least 30 minutes withoutcollapsing and preferably at least 2 hours at ambient temperature andpressure. Most preferably the foam persists until it is dry. A foamhaving rigidity means it is unlikely to collapse while rising in thefoaming tank or moving on the drainage device. Elasticity of the foamallows it to move out of the tank and over the drainage device.

A variety of soaps and metal salts of fatty acids or sulphuric acidester salts can be used to achieve foams of this kind.

Preferably the surfactant comprises a mixture of at least one metalsalts of a fatty acid or C₆ to C₁₈ aliphatic alcohol and at least onemetal salt of a fatty alkyl sulfate.

The level of surfactant is preferably in the range of from 5 to 5000 ppmand preferably from 10 to 500 ppm of liquor.

Preferably the molar ratio of fatty acid salt to fatty alkyl sulphate isin the range of from 1:1 to 1:4.

A particularly preferred surfactant component comprises a laurylsulphate salt such as sodium lauryl sulfate in combination with an acidsalt when the acid is selected from oleic, lauric and hexanoic acid. Wehave found a mixture of from 15 to 200 ppm sodium laurate and from 30 to200 ppm sodium lauryl sulphate to be particularly effective.

Preferably the weight ratio of one part sodium laurate to two partssodium lauryl sulfate and preferably 40 ppm of sodium laurate and 80 ppmof sodium lauryl sulfate is used. Another mixture of interest is sodiumoleate and sodium lauryl sulfate.

Where gas is used in the formation of foam any suitable gas may be used.Air is preferred on economic grounds however nitrogen may be ofparticular use where it is desired to minimise oxidation of metal.

In order to maximise the efficiency of the process it is preferred thatshorter residence times be used. Shorter residence times are a result ofthe increase in the volume of liquor and entraining air relative to eachother and relative to the volume of the foaming tank. By increasing thevolume flow rate of the liquor and of the foaming gas relative to thevolume of the tank residence times are reduced. By maintaining residencetimes preferably below 2 minutes and the volume ratio of air to waterabove 5:1 (preferably above 5:2) efficient throughput can be achievedwith a high proportion, preferably all, of the liquors being entrainedin the foam which overflows from the tank onto the drainage device.Volume ratios of air to water in the range of from 5:1 (preferably 5:2)to 5:3 are generally convenient.

The optimum throughput of liquor may be determined without undueexperimentation having regard to the method described herein. Thethroughput in the foaming tank may conveniently be measured in terms ofhydraulic loading which may be, for example, greater than 15 m³ /m² hand preferably in the range of 15 to 35 m³ /m² h. We have found that theprocess operates particularly efficiently with hydraulic loading ofabout 22 m³ /m³ h although the process of invention may be operatedusing a wide range of hydraulic loadings.

The required gas flow rate for a given system may be determined havingregard to the need for efficient foaming and may depend on variousfactors such as the surfactant choice. We have typically used gas flowrates of at least 25 Nm³ /m² h, for example in the range of from 30 to70 Nm³ /m² h.

Preferably in the method of the invention the foam will be allowed todrain so that at least 95% by volume of the liquor is recovered from thefoam.

The nature of the drainage device is not narrowly critical but will bedictated by its function of providing drainage of liquor while allowingthe foam to remain intact. The drainage device may comprise racks ortrays which allow the foam to progressively move under force of gravityand/or by the urging of newly created foam.

To reduce the space required drainage trays or racks may be stacked suchthat foam is progressively transferred to a lower level as it dries.

Generally it is convenient to allow the foam to dry under ambienttemperature and pressure, however variation of temperature, pressure orgas currents may be used to quicken drying if desired.

Examples of contaminants which may be collected using the method of theinvention include: metals including aluminium and heavy metals, beingmetals of atomic number of at least 21, such as Titanium Chromium,Manganese, Cobalt, Nickel, Copper, Zinc, Yttrium Zirconium, Molybdenum,Antimony, Tungsten Palladium, Silver, Cadmium, Tin, Mercury, Lead andUranium; and organic chemicals such as synthetic and natural oils.

The process of the invention has been found to be particularly suited toremoval of metals such as Aluminium, Chromium, Nickel, Copper, Zinc,Antimony, Lead and Mercury. Such metals may be in the form of theelemental metal or its compounds or ions. The organic chemical mostsuited to removal by the process of the invention generally have a lowwater solubility for example, a water solubility of less than 10 mg perliter.

In the case of heavy metal contaminants the method of the inventiongenerally provides at least 95% by weight retention of heavy metals inthe foam. The level of contaminants in the liquor prior to treatment maybe for example, in the range of from 0.1 parts per billion to 5000 partsper million.

It will be understood that levels of contaminants may be reduced byseveral passes through apparatus operating in accordance with the methodof the invention or by using two or more such apparatus in sequence.

The process of the invention may also be used in combination withconventional methods such as precipitation.

The theory of the foam collection is as follows. It is well known thatfor a material to be removed by flotation it must form a stable threephase contact at the interfacial region created by the separatesolid/liquid S/L, solid/gas S/G and liquid/gas L/G interfaces. The threecorresponding interfacial free energies are related to the contactangle, measured through the liquid phase, by Young's equation.

    γ.sub.S/G -γ.sub.S/L =γ.sub.L/G Y cos θ

For the material to adhere to the bubble, the work of adhesion places arequirement on these relative values such that

    γ.sub.L/G >γ.sub.S/G -γ.sub.L/G

For hydrophilic materials, such as most mineral ores, hydrated oxidesand hydroxides, a surface active molecule is adsorbed onto the surfaceto given a sufficient value to θ.

Bleier (1977) has shown that the property that determines ultimatelywhether a bubble and a particle heterocoalesce is this relativehydrophobicity of the solid's surface.

Both processes of co-precipitation with the adsorbing colloid (Fe(OH)₃)and adsorption on to the colloid appear important in scavenging metalions from solution. Chatman et al (1977) showed that the removal of CuIIwas predominantly by a co-operation route. While Haug (1982) has shownthat CrVI may not only be adsorbed onto the foaming Fe(OH)₃ surface butalso be mixed inside the Fe(OH)₃ precipitated by an inclusion mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described with reference to theattached drawing. In the drawing FIG. 1 is a schematic plan of atreatment plant adapted to treat waste water in accordance with thisinvention.

During operation of the plant wastewater to be treated is introduced tothe holding or pretreatment tank 1 where stirrer 2 mixes the waste waterto provide an even consistency. Wastewater is continuously fed into thefoaming tank 7 by pump 3 and surfactant is continuously pumped by pump 5into the wastewater prior to the wastewater entering the foaming tank.Foam is continuously produced from the wastewater in the tank 7 by airintroduced by pump 6 at or adjacent the bottom of the tank 7 andoverflows from the tank 7 via overflow conduit 8 onto stacked drainagetrays 9. Foam drains as it passes over the trays under the action ofgravity and the urging of newly created foam and dried foam is collectedin bin 10.

Drained liquor is recovered via conduit 11 for further treatment or safedisposal.

The invention will now be demonstrated by, but is in no way limited to,the following Examples:

EXAMPLE 1

A laboratory scale plant of the design described above with reference toFIG. 1 was used to separate heavy metals from effluent from anelectroplating installation.

The samples were all chromium stream samples which had already beentreated with metabisulfite at pH3 to reduce Cr VI to Cr III.

The level of heavy metals in these samples was in the following ranges:

Cr: 50-100 ppm

Ni: 20-70 ppm

Zn: 1-3 ppm

The extent of chromium reduction in each sample was determined bypotentiometric titration.

A wide range of degrees of metabisulfite reduction was found to occurunder normal circumstances i.e. some samples were found to contain largeexcesses of unreacted metabisulfite while others were found to containconsiderable quantities of hexavalent chromium. (This is an indicationof poor ORP probe maintenance). To ensure reproducibility between runsit was necessary to either add metabisulfite, in the samples wherechromium VI was present, or to remove metabisulfite, in samples whereexcess metabisulfite was present, by adding hydrogen peroxide. In bothcases, reactants were added such that the final level of metabisulfitewas approximately 1×10⁻⁴ mole/liter SO₃ ⁼.

25 liter batches of the wastewater were made 25 ppm wrt Fe³⁺. The pH wasthen increased to 8.0 by adding NaOH. The wastewater was then pumped tothe flotation cell. The surfactant mixture was added in the lines justprior to entry into the flotation cell. (see FIG. 1).

A foam was generated by passing air through a porous glass air diffuserat the bottom of the flotation cell.

The foam was collected in wide, shallow containers which allowdewatering and concentration of the foam product to 5% solids.

The effect the following parameters have on the heavy metal removal andon the foam product stability were studied:

feed flow rate

air flow rate

total surfactant concentration

column height

metabisulfite concentration

The apparatus depicted in FIG. 1 (2 liter flotation cell capacity) wasoperated at a number of different feed flow rates, surfactantconcentrations and air flow rates.

The treated effluent was sampled at regular intervals during each runand analysed for Cr, Ni and Zn by atomic absorption spectroscopy.

The maximum heavy metal removals achieved at the three highest feed flowrates are tabulated below. (Gas flow rate: 4.8 l/min, surfactant conc.:100 ppm)

                  TABLE 1                                                         ______________________________________                                        Maximum heavy metal removal and feed flow rate                                       Conc. of   Conc. of                                                           heavy metal                                                                              heavy metal Removal                                                                              Feed                                            in untreated                                                                             in treated  %      flow rate                                Metal  effluent(ppm)                                                                            effluent(ppm)                                                                             (average)                                                                            (l/min)                                  ______________________________________                                        Cr     80.5       4.7         94     2.0                                      Ni     58.3       6.6         89     2.0                                      Zn     2.32       0.19        92     2.0                                      Cr     74.32      2.8         96     2.5                                      Ni     52.3       2.7         95     2.5                                      Zn     2.21       0.11        95     2.5                                      Cr     80.0       1.2         98     3.0                                      Ni     55.3       3.2         94     3.0                                      Zn     3.27       0.05        98     3.0                                      ______________________________________                                    

The process is able to meet the 10 ppm limit for the disposal of thesemetals to sewer at very high feed flow rates. (Residence times of 0.7minutes in flotation cell).

The average small electroplating firm produces approximately 12,000liters of wastewater per day. The semi-bench scale rig used aboveoperated at 190 l/hr. Hence, in a 12 hour day this rig can treat 2,160liters of water.

To treat 12,000 liters of wastewater in 12 hours the full scaletreatment plant would have to be 12,000/2160=5.5 times larger i.e. aflotation cell volume of 5.5×2=11 liters would be sufficient.

The foam exiting the floatation column contains substantial quantitiesof water. Before this is disposed of, it must first be dewatered andconcentrated.

Foam drainage rate experiments revealed that wide and shallow collectionvessels (i.e. of high surface are to volume ratio) were the mosteffective in dewatering and concentrating the foam product byfacilitating the drainage process. It was found that if the fresh wetfoam was allowed to enter one end of a wide and shallow collectionvessel, by the time the foam reached the other side of the collectionvessels considerable dewatering and concentration had taken place.

Preliminary results with the 2 liter rig described above indicate thatthis mode of concentration of the foam product is extremely efficient.Using this equipment, run at 1 liter/min until steady state has beenachieved (i.e. until the velocity of the foam front in the collectiontrays was approximately zero) a collection vessel surface area of 0.3 m²was sufficient to achieve a foam product of 5% solids content.

To minimise the amount of floor space required for this process, a stacktype arrangement of trays was devised in which the foam product canprogressively overflow from higher trays to lower ones while draining,until the maximum solids content is achieved.

Table 2 illustrates the heavy metal removal for varying flow rates ofeffluent and air using as surfactant sodium laurate (NL) and sodiumlauryl sulfate (NLS).

                  TABLE 2                                                         ______________________________________                                        Percent heavy metal removal (foam drainage)                                   Surfactant conc.: 40/80 (ppm NL/ppm NLS)                                      Effluent   Gas                                                                flow rate  flow rate                                                                              Metal                                                     (ml/min)   (ml/min) Cr         Ni   Zn                                        ______________________________________                                         400        950     95         85   90                                         400       1400     95         82   92                                         400       1900     94         91   96                                         400       2300     97         91   98                                         600       2300     92         92   98                                         600       2750     98         94   96                                        1000       4500     97         94   95                                        1000       5000     95         93   94                                        1500       3600     97         93   99                                        1500       4500     99         92   99                                        ______________________________________                                    

The chemical consumption requirements for this process in relation tochromate removal are as follows.

Alkali: Both the conventional process and this invention being developeddepend on the precipitation of the heavy metal with alkali. The pH atwhich the maximum removal of heavy metals is achieved depends on thesolubilities of the heavy metal hydroxides and to some extent on whichheavy metals are present. With the conventional process, although the pHfor the maximum removal of the heavy metals may have been achieved, morealkali is added to raise the pH to the optimum pH for the flocculationstage (pH 9-10).

The flotation process, on the other hand may operate at pH 7.5-8.0 andhence does not need this additional use of alkali.

Acids: The chromic acid rinsed off freshly chrome plated items needs tobe reduced to chromium III so that precipitation can occur. Bothprocesses depend on this stage for good heavy metal removals.

This reduction is achieved by the addition of sodium metabisulfite (orSO₂) to the rinse water. This reaction is very slow at neutral pHs andhence large quantities of acid (usually sulfuric) are required to bringthe pH down to a low enough pH to allow convenient residence times intreatment tanks to be achieved (pH 3).

This costly reduction stage at acidic pHs can be replaced with reductionat near neutral pHs with a sacrifical iron electrode. i.e.

    Fe→Fe.sup.2+

    3Fe.sup.2+ +CrO.sub.4.sup.= +4H.sub.2 O→2Fe.sup.3+ +Cr.sup.3+ +8OH.sup.-

By using this mode of reduction substantial savings could be made.

The main reason for not adopting this mode of reduction with theflocculation and sedimentation process is that for every mole ofchromate in the wastewater three mole of ferric ion are produced. Hencea greater quantity of wet sludge is generated with this mode ofreduction. Reduction with a sacrificial iron electrode would be moresuited to adsorbing colloid flotation since the foam product from thisprocess can be inexpensively coverted to a dry powder and is hence lessexpensive to dispose of than a wet sludge from the flocculation andsedimentation process.

Surfactant or flocculant: The polymer flocculants used in conventionaltechnology are used at a lower level than the surfactants in theflotation process (approximately 5 ppm for flocculants compared withapproximately 60 ppm for the surfactants). This added expense associatedwith the use of surfactants would, however, be partially offset by thefact that the surfactants used are less expensive than polymerflocculants.

From the above it can be seen that the foam generation and drainageprocess of this invention overcomes some major problems associated withconventional heavy metal removal.

EXAMPLE 2

This Example demonstrates the use of the method of the invention inremoval of organic chemical contaminants.

Wastewater containing 620 mg/l fibremakers spinning oil was foamed inaccordance with the procedure of Example 1 using a batch process and thesurfactant was present as 40 ppm sodium laurate and 80 ppm sodium laurylsulfate.

The liquor drained from the foam contained 60 mg/l spinning oil.

EXAMPLE 3

This Example demonstrates the use of the invention in removal of mercuryfrom the industrial waste water.

To waste water containing 25 ppm mercury was added surfactant to provide100 ppm sodium laurate and 200 ppm sodium lauryl sulphate and aluminiumnitrate and ferrous sulphate were welded to provide 100 ppm AlIII and100 ppm FeII. The pH was adjusted to 8.5 by addition of NaOH.

The mixture was farmed according to the process of Example 1 using theapparatus of FIG. 1 and the liquor drained from the foam was found tocomprise 11 parts per billion mercury.

EXAMPLE 4

This Example demonstrates the effect of pH on the method of theinvention. The waste water mixture was prepared and treated as forExample 3 with the exception that the pH was altered by variation of theamount of NaOH added.

The efficiency of mercury removal at different pH's is tabulated below.

    ______________________________________                                                      remaining Hg                                                    pH            parts per billion                                                                         % Hg                                                ______________________________________                                        6.0           30.7        0.123                                               7.0           25.9        0.104                                               7.5           20.5        0.082                                               8.0           18.2        0.073                                               8.5           11.0        0.044                                               9.0            7.9        0.032                                               ______________________________________                                    

EXAMPLE 5

This Example demonstrates the effect of variation of colloidconcentration on mercury removal.

The process of Example 3 was repeated with varying concentration ofAlIII and FeII.

The efficiency of removal of mercury at the varying adsorbing colloidconcentrations is tabulated below.

    ______________________________________                                        AlIII    Fe'II       remaining Hg                                             (ppm)    (ppm)       parts per billion                                                                         % Hg                                         ______________________________________                                        100      100         11.0        0.044                                        150       50         1.05        0.042                                        175       25         17.2        0.069                                         50      150         7.6         0.030                                        ______________________________________                                    

EXAMPLE 6

The procedure of Example 3 was repeated using a waste water samplehaving a mercury concentration of 56 parts per billion and the removalof Hg using various adsorbing colloid concentrations is shown in thetable below.

    ______________________________________                                        AlIII    FeII        Remaining Hg                                             ppm      (ppm)       (ppb)       % Hg                                         ______________________________________                                        100      100         0.7         1.25                                         150       50         0.5         0.89                                          50      150         0.0         0.00                                         ______________________________________                                    

We claim:
 1. A method of recovering contaminants selected from the groupconsisting of aluminum, heavy metals, and organic chemicals fromsuspension or solution in a liquor which comprises feeding the liquorinto a tank, forming a foam in said tank by passing gas through theliquor in the tank such that greater than 50% by volume of the liquidfed into the tank is displaced by overflow from the tank in the form offoam onto a drainge device, wherein the liquor is foamed in the presenceof a surfactant mixture of at least one C₆ to C₁₈ aliphatic alcohol orsalt of a fatty acid and at least one salt of a fatty alkyl sulphate,and an adsorbing colloid selected from Fe(OH)₂, and Al(OH)₃ and mixturesthereof, allowing the liquor in said foam to drain and the foam to dryon said device, contaminant being retained in the dry foam, andcollecting the thus drained liquor with a reduced contaminant content.2. A method of recovering contaminants according to claim 1 whereinliquor is continuously fed into the tank, and foam is formed by thecontinuous introduction of gas into the liquor.
 3. A process accordingto claim 1 wherein at least 95% by volume of liquor fed into the tank isdisplaced by overflow from the tank in the form of foam.
 4. A method ofrecovering contaminants according to claim 1 wherein the surfactantcomprises a mixture of a fatty acid salt and fatty alkyl sulfate salt ina molar ratio of said acid to said sulphate in the range of from 1:1 to1:4.
 5. A method of recovering contaminants according to claim 1 whereinthe surfactant is a mixture of sodium laurate and sodium lauryl sulfatepresent at a level in the range of from 5 to 1000 ppm.
 6. A method ofrecovering contaminants according to claim 1 wherein the volume ratio ofgas used in foaming to liquor fed into the tank is in the range of from5:1 to 5:3.
 7. A method of recovering contaminants according to claim 1wherein the hydraulic loading of the foaming tank is in the range offrom 15 to 35 m³ /m² h and the gas flow rate into the foaming tank is atleast 25 Nm³ /m² h.
 8. A method of recovering contaminants according toclaim 7 wherein the contaminants are heavy metals.
 9. A method ofrecovering contaminants according to claim 1 wherein at least 95% byvolume of liquor is recovered from the foam.
 10. A method of recoveringcontaminants according to claim 1 wherein the contaminants compriseheavy metals and wherein the liquor drained from the foam contains lessthan 5% by weight of the contaminant level of the unfoamed liquor.